1 . Field of the Invention
The present invention relates to a two-stage railgun accelerating apparatus for projecting an object at a super high speed.
2. Description of the Prior Art
One example of a two-stage railgun accelerating apparatus in the prior art is shown in FIGS. 21 to 24, and another example thereof is shown in FIGS. 25 and 26.
First, the two-stage railgun accelerating apparatus of the prior art shown in FIGS. 21 to 24 will be described. Reference numeral 1 in FIG. 21 designates a gas gun initial accelerating apparatus, reference numeral 2 in FIGS. 21 and 22 designates an introducing pipe, reference numeral 3 designates a pulse shaping network, numeral 4 designates plasma, numeral 5 in FIGS. 21 and 24 designates a flying object, numerals 6 designate rails, numeral 8 in FIG. 23 designates a needle, reference character d.sub.1 in FIG. 24 designates an inner diameter of the introducing pipe 2 (nearly equal to an outer diameter of the flying object 5), reference character d.sub.2 designates the diameter of the space between the rails 6 on the downstream side of the introducing pipe, and reference character d.sub.3 designates the diameter of the space between the rails 6 downstream from the space on the downstream side of the introducing pipe. The needle 8 projects into the space between the rails 6 on the downstream side of the introducing pipe (the portion having an inner diameter d.sub.2).
Next, the two-stage railgun accelerating apparatus of the prior art shown in FIGS. 25 and 26 will be described. Reference numeral 2 designates an introducing pipe, numeral 5 designates a flying object, numerals 6 designate rails, numeral 8 designates a needle, reference character d.sub.1 designates an inner diameter of the introducing pipe 2 (nearly equal to an outer diameter of the flying object 5), and reference character d.sub.4 designates a diameter of the space between the rails 6 (nearly equal to an outer diameter of the flying object 5). The needle 8 is embedded and does not project into the space on the downstream side of the introducing pipe.
Now, the operations of the two-stage railgun accelerating apparatuses shown in FIGS. 21 to 24 and in FIGS. 25 and 26, respectively, will be described. The flying object 5 projected from the gas gun initial accelerating apparatus 1 is injected from the introducing pipe 2 into the space between the rails 6 while being subjected to initial acceleration by the expansion of acceleration gas. When it passes by the needle 8, a voltage is applied from a discharging power supply 10 through the needle 8 to the acceleration gas behind the flying object 5. Hence, the acceleration gas behind the flying object 5 is broken down, resulting in a transformation of the acceleration gas into plasma 4. This plasma 4 is accelerated by an electromagnetic force (Lorentz force) generated by an electric current produced by a voltage applied between the pair of rails 6 by means of the pulse shaping network 3, and by a magnetic field produced by the plasma itself, whereby the flying object 5 positioned in front of this plasma 4 is additionally accelerated.
Furthermore, the details of the rails in the prior art will be described with reference to FIG. 27. In this figure, reference numeral 01 designates rails having a nearly trapezoidal cross section in a railgun portion, numeral 02 designates insulators interposed between the respective rails 01 and having a nearly trapezoidal cross section, and numeral 03 designates seal members interposed between inclined surfaces of the respective insulators 02 and the adjacent rails 01. The rails 01 having a trapezoidal cross section and the insulators 02 having a trapezoidal cross section are alternately disposed with the seal members 03 being interposed between adjoining surfaces of these members 01 and 02. A flying object passageway having a circular cross section conformed to the cross-sectional shape of the flying object is formed within these members 01 and 02.
FIG. 28 shows another example of the flying object passageway of the railgun portion in the prior art. In this case, flat plate-shaped rails 01 and insulators 02 having a shallow T-shaped cross section are alternately disposed. Seal members 03 are interposed between the adjacent members, whereby a flying object passageway having a rectangular cross section conformed to the cross-sectional shape of the flying object is formed.
In the heretofore known two-stage railgun accelerating apparatus illustrated in FIGS. 21 to 24 there were problems in that (1) since the needle 8 projects into the space between the rails 6 at the downstream side of the introducing pipe (the portion having an inner diameter d.sub.2), the flying object 5 injected from the introducing pipe 2 into the space between the rails 6 would collide against the needle 8, resulting in a breakdown of or damage to the flying object 5, (2) since the flying object 5 having an outer diameter nearly equal to the inner diameter d.sub.1 of the introducing pipe 2 is injected into the space between the rails 6 having a larger diameter than the diameter d.sub.1 (the space having a diameter d.sub.2), the plasma would leak around the flying object 5, and hence the flying object 5 could not be additionally accelerated effectively, and (3) the probability of the flying object 5 entering the space between the rails 6 having a diameter close to the outer diameter of the flying object 5 (the space having a diameter d.sub.3) after it had been injected into the space between the rails 6 on the downstream side of the introducing pipe (the space having an inner diameter d.sub.2), was slim.
In the heretofore known type two-stage railgun accelerating apparatus illustrated in FIGS. 25 and 26, there were problems in that: (1) since the needle 8 is embedded and an electric field concentrates at the tip end of the needle 8, a uniform discharge could hardly be obtained, and (2) there was a possibility of a discharge occurring from the needle 8 towards one of the rails 6, in which case the operation would become unstable.
Another problem common to the known two-stage railgun accelerating apparatus shown in FIGS. 21 to 24 and the known two-stage railgun accelerating apparatus shown in FIGS. 25 and 26, is that because the plasma 4 is generated in the accelerating gas behind the flying object 5 by means of the needle 8, unless the pressure of the accelerating gas and the voltage applied to the needle (electrode) 8 are adjusted appropriately, the accelerating gas will not break down and a transformation of the gas into plasma 4 will not occur. As is the case with these two-stage railgun accelerating apparatuses in which the accelerating gas is transformed into plasma 4 by applying a voltage to the needle 8, the relations among the pressure of the accelerating gas, the applied voltage and the distance between the electrodes generally follow Paschen's Law (see FIG. 29). Accordingly, in the case where the distance between the electrodes is constant, unless the pressure of the accelerating gas is lowered to a certain extent, the necessary voltage to be applied would not become sufficiently low. On the contrary, if the pressure of the accelerating gas is high, the necessary voltage to be applied is high, and hence a large-capacity discharging power supply 10 becomes necessary. It is to be noted that suppressing the pressure of the accelerating gas to a low value would become a negative factor in realizing a fast flying object speed because the initial acceleration of the flying object 5 would be correspondingly low.
The above-described problems can be summarized as follows:
(i) Suppressing the applied voltage. --It is necessary to lower the pressure of the accelerating gas so that a voltage which will generate plasma can be applied. The lowering of the pressure of the gas is accompanied by a corresponding lowering of the initial speed of the flying object.
(ii) Raising the accelerating gas pressure. --It is necessary to apply a high voltage if the accelerating gas pressure is so high as to impart a sufficiently high speed to the object. --A large-capacity discharging power supply 10 is necessary to apply such a high voltage.
(iii) If the accelerating gas pressure is high, a high voltage is necessary. --A high voltage damages (erodes) the rails 6, and thus shortens the life of the rails 6.
For instance, in the case where helium (He) gas is to undergo dielectric breakdown at an interelectrode distance of 2 mm, and under a gas pressure of 50 Torr, according to Paschen's Law, the transformation of the gas into plasma can be generated by applying a dielectric breakdown voltage of about 200 V to the gas. But at this gas pressure, a sufficient initial acceleration of a flying object cannot be achieved. Accordingly, if the gas pressure is set at 5,000 Torr, that is, if the gas pressure is raised to such an extent that a sufficient initial acceleration can be achieved, then the voltage necessary for effecting dielectric breakdown is about 3,000 V, and the power supply must therefore have a large-capacity.
In addition, in the heretofore known two-stage railgun accelerating apparatus shown in FIGS. 21 to 24 as well as in the heretofore known two-stage railgun accelerating apparatus shown in FIGS. 25 and 26, in the case where the accelerating gas pressure is so low as to facilitate the generation of the plasma 4 with a moderate voltage, according to Paschen's Law dielectric breakdown is apt to occur in a selected portion of the space between the rails 6. This implies that dielectric breakdown would not occur at all portions, and consequently, there was a problem in that acceleration of the plasma 4 and of the flying object 5 could not be achieved.
Moreover, the plasma 4 generated in the above-described respective two-stage railgun accelerating apparatuses had a relatively low density and degree of ionization, and consequently, the efficiency under which the flying object 5 was accelerated was low.
Furthermore, in the heretofore known two-stage railgun accelerating apparatus shown in FIG. 27, rails 01 having a nearly trapezoidal cross section and insulators 02 having a nearly trapezoidal cross section are disposed alternately and seal members 03 are interposed between their adjoining surfaces to form a flying object passageway having a circular cross section. Hence, an inter-rail distance at the rail corner portions is small. This results in a large electric potential gradient at the corner portions and a concentration of plasma thereat. In addition, due to such reasons, electric currents would concentrate at the corner portions of the rails 01. As a result, the corner portions of the rails 01 are locally heated by Joule's heat due to the electric currents and the thermal radiation of the plasma 4, and this causes the rails to erode.
Also, in the heretofore known two-stage railgun accelerating apparatus shown in FIG. 28, flat plate-shaped rails 01 and insulators 02 having a shallow T-shaped cross section are alternately disposed, and seal members 03 are interposed between their adjoining surfaces to form a flying object passageway having a rectangular cross section. Hence, an inter-rail distance at the rail corner portions is uniform, and it seems that a concentration of plasma would hardly occur. But, because the flying object 5 has acute corner portions, a sealing of the plasma behind the flying object in the rails is poor.
In summary, in the case of the structure shown in FIG. 27, in view of erosion a current density cannot be increased. Also, in the case of the structure shown in FIG. 28, in view of the poor sealing property, a large quantity of plasma passes through the gaps between the flying object 5 and the rails 6. Therefore, both the structures shown in FIGS. 27 and 28 have a problem in that the flying object 5 cannot be additionally accelerated efficiently.
It is to be noted that if an accelerating force for the flying object 5 is represented by F, a mass of the flying object 5 is represented by m, an acceleration is represented by a, a rail inductance is represented by L, a velocity of the flying object 5 is represented by V, an accelerating time is represented by t and a current flowing through the rails and the plasma is represented by I, then the following relations are fulfilled: ##EQU1## where V.sub.o represents an initial velocity obtained by the initial acceleration.